linux64: Don't pass unnecessary -S and -g to objcopy

Since we use --input-type binary these options are rather meaningless. Both
binutils and elftoolchain ignore the option in this case, but LLVM does not,
and instead strips all symbols from the output file, causing missing symbols at
run time if building with llvm-objcopy. Thus simply remove the options; the
linux module has never included them for building its VDSO (added in r283407),
but for some reason the original commit of linux64 (r283424) added them.

These should however eventually be changed to use template assembly files as is
now done for firmware and MFS_IMAGE.

Reviewed by: emaste, trasz
Differential Revision: https://reviews.freebsd.org/D27740